1. Field of the Invention
Embodiments of the present invention generally relate to a method of tungsten film deposition, and more particularly, a method of forming a tungsten film with reduced resistivity and improved surface morphology.
2. Description of the Related Art
The semiconductor processing industry continues to strive for larger production yields while increasing the uniformity of layers deposited on substrates having larger surface areas. These goals, in combination with new materials, have led to higher integration of circuits per unit area of the substrate. As circuit integration increases, so does the need for greater uniformity and process control regarding thicknesses of layers deposited on substrates. As a result, various technologies are being developed to deposit layers on substrates in a cost-effective manner, while maintaining control over the characteristics of the layer.
Formation of film layers at a high deposition rate while providing adequate step coverage are conflicting characteristics in which one characteristic is oftentimes obtained at the expense of the other. This conflict is true particularly when refractory metal layers are deposited over gaps or vias during the formation of contacts interconnecting adjacent metallic layers separated by dielectric layers. Historically, CVD techniques have been employed to deposit conductive material such as refractory metals in order to inexpensively and quickly form contacts. With the increasing integration of semiconductor circuitry, tungsten has become a metal of choice due to its superior step coverage.
There are, however, several disadvantages to depositing tungsten by traditional CVD methods. Tungsten films are typically composite films comprising a thin tungsten nucleation layer (having a thickness of, e.g., about 50 angstrom (Å) to about 150 Å) and a thicker bulk tungsten layer (having a thickness of, e.g., about 500 Å to about 2000 Å) formed thereover. Tungsten films comprised of a thin tungsten nucleation layer and a thick bulk tungsten layer tend to have poor film morphology. Use of tungsten has frustrated photolithography steps during the manufacturing process as it results in a relatively rough surface having a reflectivity of 70% or less than that of silicon (thickness and wavelength dependent). Also, tungsten has proven difficult to deposit uniformly, and poor surface uniformity typically increases film resistivity.
Traditional tungsten bulk deposition processes applying continuous flows of tungsten-containing gas, such as tungsten hexafluoride (WF6), and reducing gas, such as hydrogen (H2), on a tungsten nucleation layer are able to achieve low resistivity but result in high surface roughness. Low resistivity is desired for better transistor device speed and low surface roughness is desired to facilitate the formation of photoresists for etching. Low resistivity tungsten is required for bitline in memory and contact in logic application. For the Reactive Ion Etch (RIE) bitline process, both low resistivity and good morphology are required.
Therefore, a need exists in the art for a method of forming tungsten films having both good film morphology and reduced resistivity.